1. Field of the Invention
The present invention relates to a solid-state imaging element typified by a light-receiving device such as a C-MOS or CCD.
2. Description of the Related Art
An area (opening portion) on a solid-state imaging device such as a CCD in which photo diodes contribute to photoelectric conversion is limited to about 20 to 40% of the total area of the solid-state imaging device, although it depends on the size and the number of pixels of the solid-state imaging device. A small opening portion directly leads to low sensitivity. In order to compensate for it, a microlens for condensing light is generally formed on a photo diode.
Recently, however, strong demands have arisen for a solid-state imaging device having a high resolution of over 3,000,000 pixels. Serious problems have been posed in terms of a reduction in the open area ratio (i.e., a reduction in the sensitivity) of a microlens attached to this high-resolution solid-state imaging device and image quality deterioration due to an increase in noise such as flare and smear. Imaging devices such as C-MOSs and CCDs have almost reached a sufficient number of pixels. Competition for the number of pixels among device makers is now changing to competition for image quality.
A known technique associated with a technique of forming microlenses is disclosed relatively in detail in, for example, Jpn. Pat. Appln. KOKAI Publication No. 60-53073. This reference discloses, in detail, a technique using the heat flow properties (heat flow) of a resin due to heat as a technique of forming a lens into a hemispherical shape and a technique of processing a lens by several etching methods. The reference also discloses, as measures against the loss of the light condensing performance of a lens surface due to light scattering, a technique of forming, on the lens surface, an organic film such as a poly(glycidylmethacrylate) (PGMA) film or an inorganic film made of OCD (an SiO2-based film formation coating solution available from Tokyo Ohka Kogyo Co., Ltd.) and the like.
A technique of forming a single-layer or multilayer antireflection film on a microlens to prevent reflection by the microlens is also disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 4-223371. In addition, a technique of dry-etching a microlens other than the above techniques is disclosed in detail in Jpn. Pat. Appln. KOKAI Publication No. 1-10666. Furthermore, a technique for chromatic microlenses (colored microlenses) is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication Nos. 64-7562 and 3-230101.
FIG. 1A is a sectional view of a typical conventional solid-state imaging device. As shown in FIG. 1A, for example, planarized layers 81 and 82, a color filter 83, and if circumstances require, an inner-layer lens are formed on a photo diode 80. As consequence, in general, an under-lens distance D1 is about 5 to 6 μm, which is relatively large (relatively thick).
FIG. 1B is a sectional view of another conventional solid-state imaging device (having chromatic lenses 90). The arrangement of the solid-state imaging device can be simplified by each chromatic lens 90 having a color filter function.
The conventional solid-state imaging devices, however, have, for example, the following problems.
First, the arrangements of the conventional solid-state imaging devices have difficulty in reducing under-lens distances. More specifically, referring to FIG. 1A, reducing (thinning) the under-lens distance D1 is a promising means for improving the condensing performance with respect to incident light from microlenses 85 and also increasing the S/N (signal-to-noise) ratio in the photo diodes 80. If, however, the thickness of each microlens 85 (lens height D2) is simply reduced, it is difficult to form a microlens into a substantially hemispherical shape by using the method of manufacturing microlenses by heat flow. Therefore, a suitable microlens cannot be manufactured.
This problem is especially obvious in a C-MOS imaging device, which has recently attracted a great deal of attention because it consumes low power and is integrated with a driving circuit to realize space saving. This is because in a C-MOS imaging device, the distance from a microlens to a photo diode tends to be large owing to its structure, and hence this arrangement is disadvantageous in reducing the
Second, with the conventional arrangement, color purity degrades to cause a deterioration in image quality depending on the incident position of light. More specifically, referring to FIG. 1B, light L1 incident near the center of the chromatic lens 90 is transmitted through a portion of the chromatic lens which has a sufficient thickness, and hence an almost intended color filter effect can be expected for transmitted light L3. In contrast to this, light L2 incident from an end portion of the chromatic lens 90 is transmitted through a thin portion of the chromatic lens serving as a color filter, and hence transmitted light L4 becomes considerably whitish. As a result, the color purity greatly degrades.